A shock absorber is a mechanical or hydraulic device designed to absorb and damp shock impulses. These shock impulses are absorbed or dampened by converting the kinetic energy of the shock impulses into thermal energy, which is then dissipated from the housing of the shock absorber.
Shock absorbers are typically separated into a compression chamber and a return chamber by a piston. A damping medium, such as hydraulic oil, is placed in the compression and return chambers and flows between the chambers through orifices in the piston. The size of the orifices in the piston are determined based on the desired dampening force of the shock absorber. In other words, the orifice size determines the pressure drop across the piston, which affects the dampening force provided by the shock absorber.
Accordingly, the pressure drop across the piston determines the pressure ratio of the shock absorber, where the pressure drop can be altered dynamically by having pressure act upon the damping medium. Such pressure can be determined by a pressurizing member mounted in or on the shock absorber body. The pressurizing member is connected to and pressurizes the compression chamber, or both the compression chamber and the return chamber. In operation, the pressurizing member is designed to receive the pressure medium that is displaced by the piston rod, to absorb the changes in damping medium volume caused by temperature differences, and to generate a certain basic pressure in the shock absorber.
In this way, shock absorbers and other dampening devices have been used to absorb shock impulses for vehicles, which are generated when vehicles are driven on uneven roads or terrain. For example, many shock absorbers or struts on vehicles utilize a piston rod that moves up and down in a cylinder to provide oscillation dampening, which provides smoothing of shock impulses that would otherwise be passed to the frame of the vehicle. Such devices typically rely upon springs, such as coil springs, disposed around the body of the shock absorber, to carry the load of the vehicle.
In this configuration, the spring internally controls a valve, where fluid within the body of the shock absorber flows in an opposing direction to the motion of the floating piston back through a two-way valve, as gas in the gas chamber decompresses or compresses in response to external circumstances, and pressure in the fluid chamber lessens or increases to restore equilibrium within the system. However, the load is only partially sustained by the compressed gas, and as a result, the device is effectively non-load-bearing without a spring.
Most shock absorbers either have a mono-tube or a twin-tube configuration. A mono-tube shock absorber includes a single, integral housing with an internal chamber including a hydraulic fluid where the chamber is separated by a floating piston. In this configuration, the chamber does not provide spring action, but rather accommodates the extra hydraulic fluid displaced by the piston rod as it moves downward within the housing during a compression stroke. Since the force created in the chamber is not enough to sustain a vehicle's weight, an external spring, as described above, is commonly added to these shock absorbers to supplement the shortage of force provided by the chamber.
A twin-tube shock absorber includes an outer cylinder and an inner cylinder that moves relative to each other. A piston rod having a piston is positioned in and reciprocally moves with the inner cylinder relative to the outer cylinder. The outer cylinder serves as a reservoir for a hydraulic fluid, such as hydraulic oil. There are fluid valves in the piston and in a stationary base valve, where the base valve controls fluid flow between both cylinders and provides some of the damping force. The valves in the piston control most of the damping in the shock absorber. In another type of twin-tube shock absorber, a gas such as low pressure Nitrogen gas is added to the shock absorber to replace oxygen air, and lessen aeration and performance fade of the hydraulic fluid.
Accordingly, there is a need for a shock absorber that provides a combination of a damping force and a spring force during both compression and extension cycles of the shock absorber.